1. Field of the Invention
The present invention relates to a registration and immobilization device for radiation therapy, and more particularly to a support device that facilitates the delivery of radiation beams from many locations and directions, and that reduces the attenuation and perturbation of radiation beams directed at the patient.
2. Description of the Related Art
Today, approximately 60% of all patients treated for cancer in the United States receive some form of radiotherapy during their course of treatment. An increasing number of patients with nonmalignant diseases also receive treatment with radiation. Examples of these diseases include arteriovenous malformations, macular degeneration, and Parkinson's disease. Radiation treatments have involved the delivery of x-rays or electrons, as well as proton or heavier ion beams. One advantage of protons and heavier ions is the precise nature in which the energy from these particles can be deposited into a patient.
For a typical treatment procedure involving the delivery of ionizing radiation, multiple beams enter the patient at different locations and from different directions. During treatment, the patient is typically placed at one or more designated locations on a patient positioner utilizing a registration device. The patient is typically immobilized with additional devices to maintain alignment of the radiation beam to the diseased target while missing critical normal tissues. Current registration and immobilization devices limit the optimization of beam entry locations and directions because they perturb the radiation beam and cause regions of the patient to receive more or less dose than prescribed. There exists a need for registration and immobilization devices that reduce perturbations in the radiation fields to acceptable levels from the desired entry directions without raising the cost of treatments prohibitively.
When a radiation treatment for a patient is designed, the dose (energy deposited per unit of mass) to non-target tissues is reduced by directing beams of radiation into the patient from multiple directions. This technique spreads the non-target dose over a large volume so that no tissue receives a large dose and critical structures receive extremely low doses. The delineation of target tissues from normal tissues and the selection of beam directions is performed with the assistance of a computerized treatment planning system (TPS). Necessary input information to the TPS can include patient images from x-ray axial computed tomography (CT). Magnetic resonance imaging (MRI) and nuclear medicine studies such as PET are also helpful in separating the diseased and normal tissues. Output from the TPS includes beam directions, shape of normal tissue shields (collimators) for each beam, and alignment information for each beam.
The patient alignment process typically comprises five separate tasks:                1. Registration—The patient is placed on a patient positioner (PP) in a reproducible manner.        2. Immobilization—The registered patient is fixated and attached to the PP so that they move together as a single unit in a controlled fashion.        3. Localization—The location of the target relative to the diagnostic, simulation, or treatment unit is determined. Typically, this is done by radiographic triangulation.        4. Positioning—The PP is moved to place the target in the desired orientation at the desired location.        5. Verification—The patient's orientation and location are verified. The technique used for this task may or may not be the same technique used for localization.        
Any of these five tasks may require iteration to succeed. When proper alignment is maintained, the margin placed around the target to account for motion can be reduced, dose to diseased tissue increased, and dose to critical tissues reduced, which in turn results in higher probability of cure with fewer complications.
Improved registration can be achieved using devices customized to the individual patient. Low-density polystyrene (e.g., Styrofoam®) has occasionally been cut to match the patient contours but this is a labor intensive procedure and is not commonly used. A faster system that is commercially available is a vinyl bag filled with polystyrene beads. The patient lies on the bag and a vacuum applied inside the bag. Once all the air is removed, the result is a hard cast of the patient. Although the beads in the bag are minimally perturbing themselves, the thick surrounding bag can perturb the beam along its edges and folds similarly to the headrest supports. The possibility of vacuum leaks is another concern with the bead bags. Yet another concern is that contamination by bodily fluids be thoroughly cleaned between patients.
After registration of a head and neck patient to the PP, the head is immobilized. This is typically accomplished with a net that covers the patient and registration device and attaches to the PP. The net is usually a sheet of the thermoplastic polyepsilon-caprolactone that has been perforated with a defined hole pattern. This plastic, with a chemical composition ((CH2)5CO2)n, becomes soft between temperatures of about 140° F. to about 180° F. and can be draped over the patient for molding to the patient's contours. The plastic will bind to itself when warm or with toluene or methyl chloride when cold. After cooling, the material becomes hard and takes on the shape of the patient. When stretched over the face and shoulders, the small holes in the sheet become large enough that the patient can breath without restriction. The finished device is often called a facemask or mask.
One major problem with masks is how to attach them to either the registration device or the PP. This is done in a secure manner such that: 1) the patient and PP move as a single unit, 2) the patient can not move during treatment, and 3) the patient is able to quickly release themselves if there is an emergency. The mask material is typically held in place between the two plastic blocks by a plurality of screws threaded through holes in both the top and bottom pieces and the thermoplastic. The frame is attached to the tabletop of the PP using cam style clamps. By reaching up and rotating the cam clamps, the ambulatory patient can perform unassisted egress. Unfortunately, this sturdy mask frame places a large block of material completely around the patient at a plane just posterior to the head. The large step in integral thickness that would be seen by radiation beams entering obliquely from below prevents the use of many desired entry angles. Some manufacturers have reduced the thickness of the frame by using thin support plates permanently bonded to the mask material using either glue or ultrasonic welding. These methods necessarily make the support plates disposable and more costly. Even these thinner support plates are often perturbing for proton and electron beams.
The table top and facemask frame also require large skin-to-aperture distances, resulting in large lateral penumbras. If a beam intercepts part of a facemask frame or tabletop, a large perturbation in the dose distribution occurs. Further, the frame and facemask attachment mechanisms prevent entry of radiation beams from a full 3π direction.
The patient positioners used in radiotherapy that the registration and immobilization devices are attached to have historically had flat tabletops for the patient to lie on. During the 1970s, some table tops were fashioned out of materials with low x-ray attenuation factors. Some table tops also had removable sections or sections with a net which allowed x-ray beams to pass relatively unimpeded to the patient. Unfortunately, the edges of these tabletops cause perturbations to the dose distributions within patients delivered by proton or electron beams. “Pod like” devices have been devised to register and immobilize the whole body of patient for various procedures. These devices contain the patient's body within the scanning region of a XCT scanner, allow beams to enter the patient with near perpendicular incidence, and decrease the need for a flat table top support through which a treatment beam passes. However, none of the devices described above have been optimized to reduce perturbations to the radiation beams nor have they had mask materials affixed directly to them.